EBI Is indeed a critical value, and all other things being more or less the same (a hundred or two hundred PCR, 2 or 3 SN, mid 60,000 gain) then a big difference in EBI would should be the decision maker.
The thing though is that most tubes I am seeing people buying are simply good all around tubes with no major weakness and no major strength, and any such tube will provide super fun viewing.
I would not get hung up in small difference in performance characteristics.
There is one exception though, and that is if the observer does much of their viewing in really hot conditions. I am such an observer. I live in Austin Tx, and on a hot summer night, my temps often don't fall below 90F until after I my sessions are done. Since my own great joy is the summer Milky Way, EBI was a huge factor in my own decision. My tubes have EBI values of .1 and .2. Now in every other way, these are non-remarkable tubes with (by astronomy standards). Photocatode is in the range of 2200, SN is in the 32 range, and gain in in the 64K range. In other words, in all other ways, these tubes are pretty average higher end L3 filmless tubes.
And that shows when Peter and I have done sessions together. Peter's tube has a higher SN, but more importantly I think, his tube has a gain of close to 70,000. When we have observed under cooler conditions (40s to 50s) his tubes have produced better views than mine. I think his EBI is in the .7-1 range, so not super good, but not at all bad. When it is cooler though, even a slight drop in EBI, combined with the very high gain (nothing to do with SN really) make for better views of galaxies.
A lot of people dismiss gain in favor of SN, and I think that is a mistake. The only time the noise is a big factor is when you are running heavy filters, and when you are running heavy filters, in my own opinion, gain and EBI are perhaps going to be more important than a super high SN.
My advice for most people is that any higher end tube with reasonably good specs will provide for a wonderful viewing experience.
I would look for SN of 30 or higher (and to me, SN is not really the most important spec) gain of 65,000 or higher, and EBI of .6 or lower. I would trade some SN for some EBI, and I would trade some SN for gain. Gain is what makes the signal stronger, and the stronger a signal is for a given amount of noise, the less the noise will interfere with it. EBI is what set the threshold, but if the gain is too low, the threshold is higher. A high gain means that a weak signal can stay above the EBI level.
Noise is to me maybe the least important attribute. I owned a PVS-7 with a gain of 32, but the EBI was 1.99. This tube was not effective for nebula observing, though it was fine for general observing. I own a PVS-7 with probably 23 to 25 gain, but the EBI is probably 1.3 or 1.4, and it works for nebula mu ch better than the tube that had SN of 32. I learned my lesson. SN by itself should not be the decision making factor.
So, EBI is important, but as long as the EBI is reasonable (ideally less than 1, but .6 or better would be even more desirable) and SN is decent (30 or above) and the gain is high (65000 or above) the tube will be enjoyable to use on all targets.
If most observing is done in colder temps, that takes the pressure off of EBI, but low EBI does set the threshold (all other things being equal) and a really low EBI is very important for threshold type work(dim nebula or dim galaxies) in hot conditions. Seeing a galaxy with a lot of scintillation in the view is in my own opinion better than not seeing the galaxy in a washed out view with less scintillation. SN over 30 will generally give excellent views even with heavy filters if the EBI is low and gain are still reasonably high. Don't sacrifice EBI and gain for a super high SN. In a perfect world, you would get everything, but in the real world, most people would not be able to have the patience for a true unicorn tube (High SN, High Gain, super low EBI).
Again, most of the people that are buying tubes are not getting super tubes, and most will say that they are very pleased. The difference from these tubes and the six month wait tubes is not all that great, and unless the EBI is pretty low after waiting six months, having a hot photocathode and high SN is not necessarily going to get you a better view than a slightly lower SN and Photocathode, but with a decently low EBI.
Edited by Eddgie, 09 June 2019 - 12:49 PM.